Currently, the development of small base stations that can be installed in a user's house, office, and the like have been advanced. 3GPP (Third Generation Partnership Project) defines such a small base station as Home Node B (HNB), and proceeds with standardization.
In 3GPP, a spreading code called PSC (Primary Scrambling Code) is defined. The PSC is defined in a range of 512 from 0 to 511. In the HNB, one cell (area where the HNB can communicate) can use only one PSC. Further, one cell must use one PSC.
A handover procedure between the HNBs agreed in 3GPP Release 9 is explained below (NPL 1). The following explanation assumes Intra HNB-GW and Intra CSG. The Intra HNB-GW indicates the state where the HNB to which a mobile station (UE) is connected (hereinafter referred to as a Source HNB) and the HNB to be a handover destination (hereinafter referred to as a Target HNB) are connected to the same HNB-GW (Home Node B Gateway). Further, the Intra CSG (Closed Subscriber Group) indicates that the Source HNB and the Target HNB are broadcasting the same CSG-ID. Note that the CSG is a method for realizing access control to the HNB defined in the 3GPP Release 8. The HNB to which this method is applied is assigned with a certain CSG-ID and broadcasts it inside its own cell. The UE accesses the HNB only when the communication to the CSG-ID is allowed.
FIG. 20 is a sequence diagram showing the handover procedure between the HNBs agreed in the 3GPP Release 9. First, the UE is in the state of communication with CS (Circuit Switch) or PS (Packet Switch) (S101).
When the UE satisfies the condition to execute handover from the HNB in communication (Source HNB) to another HNB (Target HNB), PSC used by the Target HNB is notified to the Source HNB by an RRC (Radio Resource Control):MEASUREMENT REPORT message (S102). The UE identifies the HNB by the PSC.
When the Source HNB receives the RRC:MEASUREMENT REPORT message from the UE, the Source HNB identifies the Target HNB by the PSC included in the message. Each HNB holds information (PSC, Cell ID, RNC-ID, frequency information, and the like) of neighbor HNBs. Note that the RNC-ID (Radio Network Controller Identifier) indicates an ID assigned to each HNB. Each HNB identifies the RNC-ID of the HNB which uses the received PSC according to the information held by each HNB (S103).
The Source HNB transmits an RUA (RANAP User Adaption):DIRECT TRANSFER message including the RNC-ID of the Target HNB to the HNB-GW (S104). A RANAP:(Radio Access Network Application Part) RELOCATION REQUIRED message is encapsulated in the RUA:DIRECT TRANSFER message.
Note that the RANAP is signaling exchanged between RNC and CN (Core Network) in a macro network of a related art. In a network using the HNB, it is a protocol used between the HNB and the CN. Further, the RUA is a protocol used on an Iuh interface.
The HNB-GW which received the RANAP:RELOCATION REQUIRED message identifies the Target HNB with the corresponding RNC-ID (S105). The HNB-GW transmits the RANAP:RELOCATION REQUEST message encapsulated in the RUA message to the Target HNB (S106). Note that the name of the RUA message is still not determined at the present stage. In the following explanation, the message with an undetermined name is referred to as “XXX” (for example, RUA:XXX) in each protocol specification.
When the Target HNB receives RANAP:RELOCATION REQUEST message, the Target HNB registers information on the UE that performs handover and secures a resource for the UE (S107). Then, the Target HNB transmits the RUA:DIRECT TRANSFER message encapsulating a RANAP:RELOCATION REQUEST ACKNOWLEDGE message to the HNB-GW (S108).
The HNB-GW which received the RUA:DIRECT TRANSFER message transmits a RANAP:RELOCATION COMMAND message to the Source HNB (S109). After that, a wireless synchronous process (S111) is performed between the UE and the Target HNB. Next, the Target HNB notifies that the relocation is completed to the HNB-GW (S114). Then, the Source HNB completes a series of handover processes by the HNB releasing the resource of the UE (S116).